Dynamic control of cell cycle and growth coupling by ecdysone, EGFR, and PI3K signaling in Drosophila histoblasts

The Drosophila epidermal growth factor receptor pathway regulates Hedgehog signalling and cytoneme behaviour

During Drosophila epithelial development, dynamic signalling filopodia (cytonemes) establish direct contacts between distant cells to facilitate the formation of the Hedgehog signalling gradient. However, not much is known about how cytonemes are regulated. In this study, we show that cytoneme dynamics and Hedgehog signalling in the Drosophila epithelia depend on the Epidermal Growth Factor pathway and on its downstream effector Ras1. We describe that EGFR/Ras1 pathway is required to maintain in the wing disc epithelium the basal plasma membrane levels of Interference Hedgehog (Ihog), a critical Hh co-receptor and adhesion protein. In addition, our data demonstrate that filamin A or Cheerio in Drosophila, responds to both Ihog and EGFR pathway and recruited to the basal site of the plasma membrane. This recruitment contributes to Ihog's role in stabilizing cytonemes.

Spatial regulation of Drosophila ovarian Follicle Stem Cell division rates and cell cycle transitions

Drosophila ovarian Follicle Stem Cells (FSCs) present a favorable paradigm for understanding how stem cell division and differentiation are balanced in communities where those activities are independent. FSCs also allow exploration of how this balance is integrated with spatial stem cell heterogeneity. Posterior FSCs become proliferative Follicle Cells (FCs), while anterior FSCs become quiescent Escort Cells (ECs) at about one fourth the frequency. A single stem cell can nevertheless produce both FCs and ECs because it can move between anterior and posterior locations. Studies based on EdU incorporation to approximate division rates suggested that posterior FSCs divide faster than anterior FSCs. However, direct measures of cell cycle times are required to ascertain whether FC output requires a net flow of FSCs from anterior to posterior. Here, by using live imaging and FUCCI cell-cycle reporters, we measured absolute division rates. We found that posterior FSCs cycle more than three times faster than anterior FSCs and produced sufficient new cells to match FC production. H2B-RFP dilution studies supported different cycling rates according to A/P location and facilitated live imaging, showing A/P exchange of FSCs in both directions, consistent with the dynamic equilibrium inferred from division rate measurements. Inversely graded Wnt and JAK-STAT pathway signals regulate FSC differentiation to ECs and FCs. JAK-STAT promotes both differentiation to FCs and FSC cycling, affording some coordination of these activities. When JAK-STAT signaling was manipulated to be spatially uniform, the ratio of posterior to anterior division rates was reduced but remained substantial, showing that graded JAK-STAT signaling only partly explains the graded cycling of FSCs. By using FUCCI markers, we found a prominent G2/M cycling restriction of posterior FSCs together with an A/P graded G1/S restriction, that JAK-STAT signaling promotes both G1/S and G2/M transitions, and that PI3 kinase signaling principally stimulates the G2/M transition.

Juvenile hormone suppresses sensory organ precursor determination to block Drosophila adult abdomen morphogenesis

Juvenile hormone (JH) has a classic "status quo" action at both the pupal and adult molts when administrated exogenously. In Drosophila, treatment with JH at pupariation inhibits the formation of abdominal bristles, which are derived from the histoblasts. However, the mechanism via which JH exerts this effect remains poorly understood. In this study, we analyzed the effect of JH on histoblast proliferation, migration, and differentiation. Our results indicated that whereas the proliferation and migration of histoblasts remained unaffected following treatment with a JH mimic (JHM), their differentiation, particularly the specification of sensor organ precursor (SOP) cells, was inhibited. This effect was attributable to downregulated proneural genes achaete (ac) and Scute (sc) expression levels, which prevented the specification of SOP cells in proneural clusters. Moreover, Kr-h1 was found to mediate this effect of JHM. Histoblast-specific overexpression or knockdown of Kr-h1, respectively mimicked or attenuated the effects exerted by JHM on abdominal bristle formation, SOP determination, and transcriptional regulation of ac and sc. These results indicated that the defective SOP determination was responsible for the inhibition of abdominal bristle formation by JHM, which, in turn, was mainly mediated via the transducing action of Kr-h1.

Systematic analysis of RhoGEF/GAP localizations uncovers regulators of mechanosensing and junction formation during epithelial cell division

Cell proliferation is central to epithelial tissue development, repair, and homeostasis. During cell division, small RhoGTPases control both actomyosin dynamics and cell-cell junction remodeling to faithfully segregate the genome while maintaining tissue polarity and integrity. To decipher the mechanisms of RhoGTPase spatiotemporal regulation during epithelial cell division, we generated a transgenic fluorescently tagged library for the 48 Drosophila Rho guanine exchange factors (RhoGEFs) and GTPase-activating proteins (GAPs), and we systematically characterized their endogenous distributions by time-lapse microscopy. Therefore, we unveiled candidate regulators of the interplay between actomyosin and junctional dynamics during epithelial cell division. Building on these findings, we established that the conserved RhoGEF Cysts and RhoGEF4 play sequential and distinct roles to couple cytokinesis with de novo junction formation. During ring contraction, Cysts via Rho1 participates in the neighbor mechanosensing response, promoting daughter-daughter cell membrane juxtaposition in preparation to de novo junction formation. Subsequently and upon midbody formation, RhoGEF4 via Rac acts in the dividing cell to ensure the withdrawal of the neighboring cell membranes, thus controlling de novo junction length and cell-cell arrangements upon cytokinesis. Altogether, our findings delineate how the RhoGTPases Rho and Rac are locally and temporally activated during epithelial cytokinesis, highlighting the RhoGEF/GAP library as a key resource to understand the broad range of biological processes regulated by RhoGTPases.

Tissue growth: Basement membrane degradation triggers cell proliferation

Building of the Drosophila abdomen relies on the removal of larval cells and expansion, through proliferation, of a population of progenitor epithelial cells. A new study shows that matrix metalloproteinases produced by larval cells drive basement membrane degradation and proliferative growth of the progenitor epithelial population.

Sample Preparation and Imaging of the Pupal Drosophila Abdominal Epidermis

The epithelium is one of the best studied tissues for morphogenesis, pattern formation, cell polarity, cell division, cell competition, tumorigenesis, and metastatic behaviors. However, it has been challenging to analyze real-time cell interactions or cell dynamics within the epithelia under physiological conditions. The Drosophila pupal abdominal epidermis is a model system that allows to combine long-term real-time imaging under physiological conditions with the use of powerful Drosophila genetics tools. The abdominal epidermis displays a wide range of stereotypical characteristics of the epithelia and cellular behaviors including cell division, cell death, cell rearrangement, apical constriction, and apicobasal/planar polarity, making this tissue a first choice for the study of epithelial morphogenesis and relevant phenomena. In this chapter, I describe the staging and mounting of pupae and the live imaging of the abdominal epidermis. Moreover, methods to combine live imaging with mosaic analysis or drug injection will be presented. The long-term live imaging of the pupal abdominal epidermis is straightforward and opens up the possibility to analyze cell dynamics during epithelial morphogenesis at an unprecedented resolution.

Rapid and robust optogenetic control of gene expression in Drosophila

Deciphering gene function requires the ability to control gene expression in space and time. Binary systems such as the Gal4/UAS provide a powerful means to modulate gene expression and to induce loss or gain of function. This is best exemplified in Drosophila, where the Gal4/UAS system has been critical to discover conserved mechanisms in development, physiology, neurobiology, and metabolism, to cite a few. Here we describe a transgenic light-inducible Gal4/UAS system (ShineGal4/UAS) based on Magnet photoswitches. We show that it allows efficient, rapid, and robust activation of UAS-driven transgenes in different tissues and at various developmental stages in Drosophila. Furthermore, we illustrate how ShineGal4 enables the generation of gain and loss-of-function phenotypes at animal, organ, and cellular levels. Thanks to the large repertoire of UAS-driven transgenes, ShineGal4 enriches the Drosophila genetic toolkit by allowing in vivo control of gene expression with high temporal and spatial resolutions.

Multiple Wnts act synergistically to induce Chk1/Grapes expression and mediate G2 arrest in Drosophila tracheoblasts

Larval tracheae of Drosophila harbour progenitors of the adult tracheal system (tracheoblasts). Thoracic tracheoblasts are arrested in the G2 phase of the cell cycle in an ATR (mei-41)-Checkpoint Kinase1 (grapes, Chk1) dependent manner prior to mitotic re-entry. Here we investigate developmental regulation of Chk1 activation. We report that Wnt signaling is high in tracheoblasts and this is necessary for high levels of activated (phosphorylated) Chk1. We find that canonical Wnt signaling facilitates this by transcriptional upregulation of Chk1 expression in cells that have ATR kinase activity. Wnt signaling is dependent on four Wnts (Wg, Wnt5, 6,10) that are expressed at high levels in arrested tracheoblasts and are downregulated at mitotic re-entry. Interestingly, none of the Wnts are dispensable and act synergistically to induce Chk1. Finally, we show that downregulation of Wnt signaling and Chk1 expression leads to mitotic re-entry and the concomitant upregulation of Dpp signaling, driving tracheoblast proliferation.

Rhodnius, Golden Oil, and Met: A History of Juvenile Hormone Research

Juvenile hormone (JH) is a unique sesquiterpenoid hormone which regulates both insect metamorphosis and insect reproduction. It also may be utilized by some insects to mediate polyphenisms and other life history events that are environmentally regulated. This article details the history of the research on this versatile hormone that began with studies by V. B. Wigglesworth on the "kissing bug" Rhodnius prolixus in 1934, through the discovery of a natural source of JH in the abdomen of male Hyalophora cecropia moths by C. M. Williams that allowed its isolation ("golden oil") and identification, to the recent research on its receptor, termed Methoprene-tolerant (Met). Our present knowledge of cellular actions of JH in metamorphosis springs primarily from studies on Rhodnius and the tobacco hornworm Manduca sexta, with recent studies on the flour beetle Tribolium castaneum, the silkworm Bombyx mori, and the fruit fly Drosophila melanogaster contributing to the molecular understanding of these actions. Many questions still need to be resolved including the molecular basis of competence to metamorphose, differential tissue responses to JH, and the interaction of nutrition and other environmental signals regulating JH synthesis and degradation.

Mechanical coordination is sufficient to promote tissue replacement during metamorphosis in Drosophila

During development, cells coordinate to organize in coherent structures. Although it is now well established that physical forces are essential for implementing this coordination, the instructive roles of mechanical inputs are not clear. Here, we show that the replacement of the larval epithelia by the adult one in Drosophila demands the coordinated exchange of mechanical signals between two cell types, the histoblasts (adult precursors) organized in nests and the surrounding larval epidermal cells (LECs). An increasing stress gradient develops from the center of the nests toward the LECs as a result of the forces generated by histoblasts as they proliferate and by the LECs as they delaminate (push/pull coordination). This asymmetric radial coordination of expansive and contractile activities contributes to epithelial replacement. Our analyses support a model in which cell-cell mechanical communication is sufficient for the rearrangements that implement epithelial morphogenesis.

Negative regulation of G2-M by ATR (mei-41)/Chk1(Grapes) facilitates tracheoblast growth and tracheal hypertrophy in Drosophila

Imaginal progenitors in Drosophila are known to arrest in G2 during larval stages and proliferate thereafter. Here we investigate the mechanism and implications of G2 arrest in progenitors of the adult thoracic tracheal epithelium (tracheoblasts). We report that tracheoblasts pause in G2 for ~48–56 h and grow in size over this period. Surprisingly, tracheoblasts arrested in G2 express drivers of G2-M like Cdc25/String (Stg). We find that mechanisms that prevent G2-M are also in place in this interval. Tracheoblasts activate Checkpoint Kinase 1/Grapes (Chk1/Grp) in an ATR/mei-41-dependent manner. Loss of ATR/Chk1 led to precocious mitotic entry ~24–32 h earlier. These divisions were apparently normal as there was no evidence of increased DNA damage or cell death. However, induction of precocious mitoses impaired growth of tracheoblasts and the tracheae they comprise. We propose that ATR/Chk1 negatively regulate G2-M in developing tracheoblasts and that G2 arrest facilitates cellular and hypertrophic organ growth.

Every organism begins as a single cell. That cell, and all the other cells it generates over time, need to divide at the right time and in the right place to develop into an adult. As they do so, they pass through the stages of the cell cycle. As cells prepare to divide they enter into the first growth phase, G1, ramping up their metabolic activity. They then enter S phase, duplicate their DNA, and subsequently a second growth phase G2. Finally, during the mitotic phase, the chromosome separate and cells undergo cytokinesis to form new cells.

Dividing cells can pause at certain stages of the cell cycle to assess whether the conditions are suitable to proceed. The length of the pause depends on the stage of development and the cell type. Signals around the cell provide the cues that it needs to make the decision.

The fruit fly Drosophila melanogaster, for example, undergoes metamorphosis during development, meaning it transforms from a larva into an adult. The larva contains small patches of ‘progenitor’ cells that form the adult tissue. These remain paused for various intervals during larval life and restart their cell cycle as the animal develops. A key challenge in biology is to understand how these progenitors pause and what makes them start dividing again. Here, Kizhedathu, Bagul and Guha uncover a new mechanism that pauses the cell cycle in developing animal cells.

Progenitors of the respiratory system in the adult fruit fly pause at the G2 stage of the cell cycle during larval life. Some of these progenitors, from a part of the larva called the dorsal trunk, go on to form the structures of the adult respiratory system. By counting the cells and tracking their dynamics with fluorescent labels, Kizhedathu et al. revealed that the progenitor cells pause for between 48 and to 56 hours. Previous research suggested that this pause happens because the cells lack a protein essential for mitosis called Cdc25/String. However, these progenitors were producing Cdc25/String. They stopped dividing because they also made another protein, known as Checkpoint Kinase 1/Grapes (Chk1/Grp).

Chk1 is known to add a chemical modification to Cdc25, which dampens its activity and stops the cell cycle from progressing. This is likely what allow the flies to co-ordinate their development and give the cells more time to grow. When Chk1 was experimentally removed, it reactivated the paused cells sooner, resulting in smaller cells and a smaller respiratory organ.

This work extends our understanding of stem cell dynamics and growth during development. Previous work has shown that cells that give rise to muscles and the neural tube (the precursor of the central nervous system) also pause their cell cycle in G2. Understanding more about how this happens could open new avenues for research into developmental disease.

Ras/ERK-signalling promotes tRNA synthesis and growth via the RNA polymerase III repressor Maf1 in Drosophila

The small G-protein Ras is a conserved regulator of cell and tissue growth. These effects of Ras are mediated largely through activation of a canonical RAF-MEK-ERK kinase cascade. An important challenge is to identify how this Ras/ERK pathway alters cellular metabolism to drive growth. Here we report on stimulation of RNA polymerase III (Pol III)-mediated tRNA synthesis as a growth effector of Ras/ERK signalling in Drosophila. We find that activation of Ras/ERK signalling promotes tRNA synthesis both in vivo and in cultured Drosophila S2 cells. We also show that Pol III function is required for Ras/ERK signalling to drive proliferation in both epithelial and stem cells in Drosophila tissues. We find that the transcription factor Myc is required but not sufficient for Ras-mediated stimulation of tRNA synthesis. Instead we show that Ras signalling promotes Pol III function and tRNA synthesis by phosphorylating, and inhibiting the nuclear localization and function of the Pol III repressor Maf1. We propose that inhibition of Maf1 and stimulation of tRNA synthesis is one way by which Ras signalling enhances protein synthesis to promote cell and tissue growth.

The Hippo transducer TAZ promotes cell proliferation and tumor formation of glioblastoma cells through EGFR pathway

TAZ, a WW-domain-containing transcriptional co-activator, is important for development of various tissues in mammals. Recently, TAZ has been found to be overexpressed in some types of human cancers. However, the role of TAZ in glioblastoma remains unclear. In this study, we found that TAZ was overexpressed in prognostically poor glioblastoma patients. Through knocking down or overexpressing TAZ in U87 and LN229 cells, the expression level of TAZ was found to be positively related to cell proliferation in vitro and tumor formation in vivo. Further investigation indicated that TAZ could significantly promote the acceleration of cell cycle. Moreover, the western blot for p-EGFR, p-AKT, p-ERK1/2, p21, cyclin E and CDK2 proteins, target genes of the EGFR pathway, indicated that TAZ significantly activated EGFR/AKT/ERK signaling. Additionally, the blockage of EGFR pathway resulted in a significantly inhibition of cell proliferation induced by TAZ. Taken together, these results demonstrate that TAZ can promote proliferation and tumor formation in glioblastoma cells by potentiating the EGFR/AKT/ERK pathway, and provide the evidence for promising target for the treatment of glioblastoma.

Differentiation of lepidoptera scale cells from epidermal stem cells followed by ecdysone-regulated DNA duplication and scale secreting

Integuments are the first line to protect insects from physical damage and pathogenic infection. In lepidopteran insects, they undergo distinct morphology changes such as scale formation during metamorphosis. However, we know little about integument development and scale formation during this stage. Here, we use the silkworm, Bombyx mori, as a model and show that stem cells in the integument of each segment, but not intersegmental membrane, divide into two scale precursor cells during the spinning stage. In young pupae, the scale precursor cell divides again. One of the daughter cells becomes a mature scale-secreting cell that undergoes several rounds of DNA duplication and the other daughter cell undergoes apoptosis later on. This scale precursor cell division is crucial to the development and differentiation of scale-secreting cells because scale production can be blocked after treatment with the cell division inhibitor paclitaxel. Subsequently, the growth of scale-secreting cells is under the control of 20-hydroxyecdysone but not juvenile hormone since injection of 20-hydroxyecdysone inhibited scale formation. Further work demonstrated that 20-hydroxyecdysone injection inhibits DNA duplication in scale-secreting cells while the expression of scale-forming gene ASH1 was down-regulated by BR-C Z2. Therefore, this research demonstrates that the scale cells of the silkworm develops through stem cell division prior to pupation and then another wave of cell division differentiates these cells into scale secreting cells soon after entrance into the pupal stage. Additionally, DNA duplication and scale production in the scale-secreting cells were found to be under the regulation of 20-hydroxyecdysone.

A population of G2-arrested cells are selected as sensory organ precursors for the interommatidial bristles of the Drosophila eye

Cell cycle progression and differentiation are highly coordinated during the development of multicellular organisms. The mechanisms by which these processes are coordinated and how their coordination contributes to normal development are not fully understood. Here, we determine the developmental fate of a population of precursor cells in the developing Drosophila melanogaster retina that arrest in G2 phase of the cell cycle and investigate whether cell cycle phase-specific arrest influences the fate of these cells. We demonstrate that retinal precursor cells that arrest in G2 during larval development are selected as sensory organ precursors (SOPs) during pupal development and undergo two cell divisions to generate the four-cell interommatidial mechanosensory bristles. While G2 arrest is not required for bristle development, preventing G2 arrest results in incorrect bristle positioning in the adult eye. We conclude that G2-arrested cells provide a positional cue during development to ensure proper spacing of bristles in the eye. Our results suggest that the control of cell cycle progression refines cell fate decisions and that the relationship between these two processes is not necessarily deterministic.

Snipper, an Eri1 homologue, affects histone mRNA abundance and is crucial for normal Drosophila melanogaster development

The conserved 3'-5' RNA exonuclease ERI1 is implicated in RNA interference inhibition, 5.8S rRNA maturation and histone mRNA maturation and turnover. The single ERI1 homologue in Drosophila melanogaster Snipper (Snp) is a 3'-5' exonuclease, but its in vivo function remains elusive. Here, we report Snp requirement for normal Drosophila development, since its perturbation leads to larval arrest and tissue-specific downregulation results in abnormal tissue development. Additionally, Snp directly interacts with histone mRNA, and its depletion results in drastic reduction in histone transcript levels. We propose that Snp protects the 3'-ends of histone mRNAs and upon its absence, histone transcripts are readily degraded. This in turn may lead to cell cycle delay or arrest, causing growth arrest and developmental perturbations.

Ecdysone signaling induces two phases of cell cycle exit in Drosophila cells

During development, cell proliferation and differentiation must be tightly coordinated to ensure proper tissue morphogenesis. Because steroid hormones are central regulators of developmental timing, understanding the links between steroid hormone signaling and cell proliferation is crucial to understanding the molecular basis of morphogenesis. Here we examined the mechanism by which the steroid hormone ecdysone regulates the cell cycle in Drosophila. We find that a cell cycle arrest induced by ecdysone in Drosophila cell culture is analogous to a G2 cell cycle arrest observed in the early pupa wing. We show that in the wing, ecdysone signaling at the larva-to-puparium transition induces Broad which in turn represses the cdc25c phosphatase String. The repression of String generates a temporary G2 arrest that synchronizes the cell cycle in the wing epithelium during early pupa wing elongation and flattening. As ecdysone levels decline after the larva-to-puparium pulse during early metamorphosis, Broad expression plummets, allowing String to become re-activated, which promotes rapid G2/M progression and a subsequent synchronized final cell cycle in the wing. In this manner, pulses of ecdysone can both synchronize the final cell cycle and promote the coordinated acquisition of terminal differentiation characteristics in the wing.

Summary: Pulsed ecdysone signaling remodels cell cycle dynamics, causing distinct primary and secondary cell cycle arrests in Drosophila cells, analogous to those observed in the wing during metamorphosis.

Drosophila insulin-like peptide 1 (DILP1) is transiently expressed during non-feeding stages and reproductive dormancy

The insulin/insulin-like growth factor signaling pathway is evolutionarily conserved in animals, and is part of nutrient-sensing mechanisms that control growth, metabolism, reproduction, stress responses, and lifespan. In Drosophila, eight insulin-like peptides (DILP1-8) are known, six of which have been investigated in some detail, whereas expression and functions of DILP1 and DILP4 remain enigmatic. Here we demonstrate that dilp1/DILP1 is transiently expressed in brain insulin producing cells (IPCs) from early pupa until a few days of adult life. However, in adult female flies where diapause is triggered by low temperature and short days, within a time window 0–10h post-eclosion, the dilp1/DILP1 expression remains high for at least 9 weeks. The dilp1 mRNA level is increased in dilp2, 3, 5 and dilp6 mutant flies, indicating feedback regulation. Furthermore, the DILP1 expression in IPCs is regulated by short neuropeptide F, juvenile hormone and presence of larval adipocytes. Male dilp1 mutant flies display increased lifespan and reduced starvation resistance, whereas in female dilp1 mutants oviposition is reduced. Thus, DILP1 is expressed in non-feeding stages and in diapausing flies, is under feedback regulation and appears to play sex-specific functional roles.

Your neighbours matter - non-autonomous control of apoptosis in development and disease

Traditionally, the regulation of apoptosis has been thought of as an autonomous process in which the dying cell dictates its own demise. However, emerging studies in genetically tractable multicellular organisms, such as Caenorhabditis elegans and Drosophila, have revealed that death is often a communal event. Here, we review the current literature on non-autonomous mechanisms governing apoptosis in multiple cellular contexts. The importance of the cellular community in dictating the funeral arrangements of apoptotic cells has profound implications in development and disease.

G2 phase arrest prevents bristle progenitor self-renewal and synchronizes cell division with cell fate differentiation

Developmentally regulated cell cycle arrest is a fundamental feature of neurogenesis, whose significance is poorly understood. During Drosophila sensory organ (SO) development, primary progenitor (pI) cells arrest in G2 phase for precisely defined periods. Upon re-entering the cell cycle in response to developmental signals, these G2-arrested precursor cells divide and generate specialized neuronal and non-neuronal cells. To study how G2 phase arrest affects SO lineage specification, we forced pI cells to divide prematurely. This produced SOs with normal neuronal lineages but supernumerary non-neuronal cell types because prematurely dividing pI cells generate a secondary pI cell that produces a complete SO and an external precursor cell that undergoes amplification divisions. pI cells are therefore able to undergo self-renewal before transit to a terminal mode of division. Regulation of G2 phase arrest thus serves a dual role in SO development: preventing progenitor self-renewal and synchronizing cell division with developmental signals. Cell cycle arrest in G2 phase temporally coordinates the precursor cell proliferation potential with terminal cell fate determination to ensure formation of organs with a normal set of sensory cells.

miR-965 controls cell proliferation and migration during tissue morphogenesis in the Drosophila abdomen

Formation of the Drosophila adult abdomen involves a process of tissue replacement in which larval epidermal cells are replaced by adult cells. The progenitors of the adult epidermis are specified during embryogenesis and, unlike the imaginal discs that make up the thoracic and head segments, they remain quiescent during larval development. During pupal development, the abdominal histoblast cells proliferate and migrate to replace the larval epidermis. Here, we provide evidence that the microRNA, miR-965, acts via string and wingless to control histoblast proliferation and migration. Ecdysone signaling downregulates miR-965 at the onset of pupariation, linking activation of the histoblast nests to the hormonal control of metamorphosis. Replacement of the larval epidermis by adult epidermal progenitors involves regulation of both cell-intrinsic events and cell communication. By regulating both cell proliferation and cell migration, miR-965 contributes to the robustness of this morphogenetic system.

HDAC9 promotes glioblastoma growth via TAZ-mediated EGFR pathway activation

Histone deacetylase 9 (HDAC9), a member of class II HDACs, regulates a wide variety of normal and abnormal physiological functions. We found that HDAC9 is over-expressed in prognostically poor glioblastoma patients. Knockdown HDAC9 decreased proliferation in vitro and tumor formation in vivo. HDAC9 accelerated cell cycle in part by potentiating the EGFR signaling pathway. Also, HDAC9 interacted with TAZ, a key downstream effector of Hippo pathway. Knockdown of HDAC9 decreased the expression of TAZ. We found that overexpressed TAZ in HDAC9-knockdown cells abrogated the effects induced by HDAC9 silencing both in vitro and in vivo. We demonstrated that HDAC9 promotes tumor formation of glioblastoma via TAZ-mediated EGFR pathway activation, and provide the evidence for promising target for the treatment of glioblastoma.

Injury response checkpoint and developmental timing in insects

In insects, localized tissue injury often leads to global (organism-wide) delays in development and retarded metamorphosis. In Drosophila, for example, injuries to the larval imaginal discs can retard pupariation and prolong metamorphosis. Injuries induced by treatments such as radiation, mechanical damage and induction of localized cell death can trigger similar delays. In most cases, the duration of the developmental delay appears to be correlated with the extent of damage, but the effect is also sensitive to the developmental stage of the treated animal. The proximate cause of the delays is likely a disruption of the ecdysone signaling pathway, but the intermediate steps leading from tissue injury and/or regeneration to that disruption remain unknown. Here, we review the evidence for injury-induced developmental delays, and for a checkpoint or checkpoints associated with the temporal progression of development and the on-going efforts to define the mechanisms involved.

Ecdysone receptor (EcR) is involved in the transcription of cell cycle genes in the silkworm

EcR (ecdysone receptor)-mediated ecdysone signaling pathway contributes to regulate the transcription of genes involved in various processes during insect development. In this work, we detected the expression of EcR gene in silkworm ovary-derived BmN4 cells and found that EcR RNAi result in an alteration of cell shape, indicating that EcR may orchestrate cell cycle progression. EcR RNAi and EcR overexpression analysis revealed that in the cultured BmN4 cells, EcR respectively promoted and suppressed the transcription of E2F-1 and CycE, two genes controlling cell cycle progression. Further examination demonstrated that ecdysone application in BmN4 cells not only changed the transcription of these two cell cycle genes like that under EcR overexpression, but also induced cell cycle arrest at G2/M phase. In vivo analysis confirmed that E2F-1 expression was elevated in silk gland of silkworm larvae after ecdysone application, which is same as its response to ecdysone in BmN4 cells. However, ecdysone also promotes CycE transcription in silk gland, and this is converse with the observation in BmN4 cells. These results provide new insights into understanding the roles of EcR-mediated ecdysone signaling in the regulation of cell cycle.

Loss of SPARC dysregulates basal lamina assembly to disrupt larval fat body homeostasis in Drosophila melanogaster

BACKGROUND

SPARC is a collagen-binding glycoprotein whose functions during early development are unknown. We previously reported that SPARC is expressed in Drosophila by hemocytes and the fat body (FB) and enriched in basal laminae (BL) surrounding tissues, including adipocytes. We sought to explore if SPARC is required for proper BL assembly in the FB.

RESULTS

SPARC deficiency leads to larval lethality, associated with remodeling of the FB. In the absence of SPARC, FB polygonal adipocytes assume a spherical morphology. Loss-of-function clonal analyses revealed a cell-autonomous accumulation of BL components around mutant cells that include collagen IV (Col lV), Laminin, and Perlecan. Ultrastructural analyses indicate SPARC-deficient adipocytes are surrounded by an aberrant accumulation of a fibrous extracellular matrix.

CONCLUSIONS

Our data indicate a critical requirement for SPARC for the proper BL assembly in Drosophila FB. Since Col IV within the BL is a prime determinant of cell shape, the rounded appearance of SPARC-deficient adipocytes is due to aberrant assembly of Col IV.

TRAF6 is a novel regulator of Notch signaling in Drosophila melanogaster

Notch signaling pathway unravels a fundamental cellular communication system that plays an elemental role in development. It is evident from different studies that the outcome of Notch signaling depends on signal strength, timing, cell type, and cellular context. Since Notch signaling affects a spectrum of cellular activity at various developmental stages by reorganizing itself in more than one way to produce different intensities in the signaling output, it is important to understand the context dependent complexity of Notch signaling and different routes of its regulation. We identified, TRAF6 (Drosophila homolog of mammalian TRAF6) as an interacting partner of Notch intracellular domain (Notch-ICD). TRAF6 genetically interacts with Notch pathway components in trans-heterozygous combinations. Immunocytochemical analysis shows that TRAF6 co-localizes with Notch in Drosophila third instar larval tissues. Our genetic interaction data suggests that the loss-of-function of TRAF6 leads to the rescue of previously identified Kurtz-Deltex mediated wing notching phenotype and enhances Notch protein survival. Co-expression of TRAF6 and Deltex results in depletion of Notch in the larval wing discs and down-regulates Notch targets, Wingless and Cut. Taken together, our results suggest that TRAF6 may function as a negative regulator of Notch signaling.

dLin52 is crucial for dE2F and dRBF mediated transcriptional regulation of pro-apoptotic gene hid

Drosophila lin52 (dlin52) is a member of Myb transcription regulator complex and it shows a dynamic pattern of expression in all Drosophila tissues. Myb complex functions to activate or repress transcription in a site-specific manner; however, the detailed mechanism is yet to be clearly understood. Members of the Drosophila melanogaster Myb-MuvB/dREAM complex have been known to regulate expression of a wide range of genes including those involved in regulating apoptosis. E2F and its corepressor RBF also belong to this complex and together they regulate expression of genes involved in cell cycle progression, apoptosis, differentiation, and development. In the present study, we examined whether the depletion of dlin52 in developing photoreceptor neurons results in enhanced apoptosis and disorganisation of the ommatidia. Strikingly, we found that dLin52 is essential for transcriptional repression of the pro-apoptotic gene, hid; decrease in dlin52 levels led to dramatic induction of hid and apoptosis in eye-antennal discs. Reduction of Rpd3 (HDAC1), another member of the dREAM complex, also led to marginal upregulation of Hid. In addition, we also demonstrated that an optimum level of dLin52 is needed for dE2F1/2 activity on the hid promoter. dlin52 cooperates with dRBF and dE2F1/2 for recruitment of repressor complex on the hid promoter. Preliminary data indicate that Rpd3/HDAC1 also contributes to hid repression. Based on the findings, we conclude that dLin52 functions as a co-factor and modulates activity of members of dMyb/dREAM complex at hid promoter, thus regulating apoptosis by repressing this pro-apoptotic gene in the developing Drosophila eye.

Xenobiotic effects on intestinal stem cell proliferation in adult honey bee (Apis mellifera L) workers

The causes of the current global decline in honey bee health are unknown. One major group of hypotheses invokes the pesticides and other xenobiotics to which this important pollinator species is often exposed. Most studies have focused on mortality or behavioral deficiencies in exposed honey bees while neglecting other biological functions and target organs. The midgut epithelium of honey bees presents an important interface between the insect and its environment. It is maintained by proliferation of intestinal stem cells throughout the adult life of honey bees. We used caged honey bees to test multiple xenobiotics for effects on the replicative activity of the intestinal stem cells under laboratory conditions. Most of the tested compounds did not alter the replicative activity of intestinal stem cells. However, colchicine, methoxyfenozide, tetracycline, and a combination of coumaphos and tau-fluvalinate significantly affected proliferation rate. All substances except methoxyfenozide decreased proliferation rate. Thus, the results indicate that some xenobiotics frequently used in apiculture and known to accumulate in honey bee hives may have hitherto unknown physiological effects. The nutritional status and the susceptibility to pathogens of honey bees could be compromised by the impacts of xenobiotics on the maintenance of the midgut epithelium. This study contributes to a growing body of evidence that more comprehensive testing of xenobiotics may be required before novel or existing compounds can be considered safe for honey bees and other non-target species.

The Ecdysone receptor constrains wingless expression to pattern cell cycle across the Drosophila wing margin in a Cyclin B-dependent manner

Background

Ecdysone triggers transcriptional changes via the ecdysone receptor (EcR) to coordinate developmental programs of apoptosis, cell cycle and differentiation. Data suggests EcR affects cell cycle gene expression indirectly and here we identify Wingless as an intermediary factor linking EcR to cell cycle.

Results

We demonstrate EcR patterns cell cycle across the presumptive Drosophila wing margin by constraining wg transcription to modulate CycB expression, but not the previously identified Wg-targets dMyc or Stg. Furthermore co-knockdown of Wg restores CycB patterning in EcR knockdown clones. Wg is not a direct target of EcR, rather we demonstrate that repression of Wg by EcR is likely mediated by direct interaction between the EcR-responsive zinc finger transcription factor Crol and the wg promoter.

Conclusions

Thus we elucidate a critical mechanism potentially connecting ecdysone with patterning signals to ensure correct timing of cell cycle exit and differentiation during margin wing development.

Cyclin E controls Drosophila female germline stem cell maintenance independently of its role in proliferation by modulating responsiveness to niche signals

Stem cells must proliferate while maintaining 'stemness'; however, much remains to be learned about how factors that control the division of stem cells influence their identity. Multiple stem cell types display cell cycles with short G1 phases, thought to minimize susceptibility to differentiation factors. Drosophila female germline stem cells (GSCs) have short G1 and long G2 phases, and diet-dependent systemic factors often modulate G2. We previously observed that Cyclin E (CycE), a known G1/S regulator, is atypically expressed in GSCs during G2/M; however, it remained unclear whether CycE has cell cycle-independent roles in GSCs or whether it acts exclusively by modulating the cell cycle. In this study, we detected CycE activity during G2/M, reflecting its altered expression pattern, and showed that CycE and its canonical partner, Cyclin-dependent kinase 2 (Cdk2), are required not only for GSC proliferation, but also for GSC maintenance. In genetic mosaics, CycE- and Cdk2-deficient GSCs are rapidly lost from the niche, remain arrested in a G1-like state, and undergo excessive growth and incomplete differentiation. However, we found that CycE controls GSC maintenance independently of its role in the cell cycle; GSCs harboring specific hypomorphic CycE mutations are not efficiently maintained despite normal proliferation rates. Finally, CycE-deficient GSCs have an impaired response to niche bone morphogenetic protein signals that are required for GSC self-renewal, suggesting that CycE modulates niche-GSC communication. Taken together, these results show unequivocally that the roles of CycE/Cdk2 in GSC division cycle regulation and GSC maintenance are separable, and thus potentially involve distinct sets of phosphorylation targets.

Homeotic functions of the Teashirt transcription factor during adult Drosophila development

During Drosophila development region-specific regulation of target genes by Hox proteins is modulated by genetic interactions with various cofactors and genetic collaborators. During embryogenesis one such modulator of Hox target specificity is the zinc-finger transcription factor Teashirt (Tsh) that is expressed in the developing trunk and cooperatively functions with trunk-specific Hox proteins to promote appropriate segment fate. This embryonic function of Tsh is characterized as homeotic since loss of embryonic Tsh activity leads to transformation of trunk segments toward head identity. In addition to this embryonic homeotic role, Tsh also performs vital Hox-independent functions through patterning numerous embryonic, larval and adult structures. Here we address whether the homeotic function of Tsh is maintained throughout development by investigating its contribution to patterning the adult abdomen. We show that Tsh is expressed throughout the developing abdomen and that this expression is dependent on the three Bithorax Hox proteins Ultrabithorax, Abdominal-A and Abdominal-B. Conditional reduction of Tsh activity during pupation reveals broad homeotic roles for this transcription factor throughout the adult abdomen. Additionally we show that, as during embryogenesis, the tsh paralog tiptop (tio) plays a partially redundant role in this homeotic activity.

Abdominal segment reduction: development and evolution of a deeply fixed trait

When a new student first begins to push flies, an immediate skill that must be learned is sorting the sexes. In Drosophila melanogaster several sexually dimorphic characters can be used to readily distinguish males from females including abdominal pigmentation, male sex combs and genital morphology. Another, often-overlooked, sexual dimorphism is adult abdominal segment number. Externally, adult Drosophila males possess one fewer abdominal segment than females; the terminal pre-genital segment apparently either absent or fused with the next-most anterior segment. Beyond known roles for the homeotic protein Abdominal-B (Abd-B) and the sex-determining transcription factor Doublesex (Dsx) as key regulators of this trait, surprisingly little is known about either the morphogenetic processes or the downstream genetics responsible for patterning these events. We have explored both and found that rapid epithelial reorganization during pupation eliminates a nascent terminal male segment. We found this Abd-B-dependent process results from sex- and segment-specific regulation of diverse developmental targets including the wingless gene and surprisingly, dsx itself. ( 1) (,) ( 2) Here, I review our observations and discuss this trait as a model to explore both dynamics of epithelial morphogenesis as well as the evolution of developmental mechanisms.

Drosophila Hox and sex-determination genes control segment elimination through EGFR and extramacrochetae activity

The formation or suppression of particular structures is a major change occurring in development and evolution. One example of such change is the absence of the seventh abdominal segment (A7) in Drosophila males. We show here that there is a down-regulation of EGFR activity and fewer histoblasts in the male A7 in early pupae. If this activity is elevated, cell number increases and a small segment develops in the adult. At later pupal stages, the remaining precursors of the A7 are extruded under the epithelium. This extrusion requires the up-regulation of the HLH protein Extramacrochetae and correlates with high levels of spaghetti-squash, the gene encoding the regulatory light chain of the non-muscle myosin II. The Hox gene Abdominal-B controls both the down-regulation of spitz, a ligand of the EGFR pathway, and the up-regulation of extramacrochetae, and also regulates the transcription of the sex-determining gene doublesex. The male Doublesex protein, in turn, controls extramacrochetae and spaghetti-squash expression. In females, the EGFR pathway is also down-regulated in the A7 but extramacrochetae and spaghetti-squash are not up-regulated and extrusion of precursor cells is almost absent. Our results show the complex orchestration of cellular and genetic events that lead to this important sexually dimorphic character change.

Quiescence: early evolutionary origins and universality do not imply uniformity

Cell cycle investigations have focused on relentless exponential proliferation of cells, an unsustainable situation in nature. Proliferation of cells, whether microbial or metazoan, is interrupted by periods of quiescence. The vast majority of cells in an adult metazoan lie quiescent. As disruptions in this quiescence are at the foundation of cancer, it will be important for the field to turn its attention to the mechanisms regulating quiescence. While often presented as a single topic, there are multiple forms of quiescence each with complex inputs, some of which are tied to conceptually challenging aspects of metazoan regulation such as size control. In an effort to expose the enormity of the challenge, I describe the differing biological purposes of quiescence, and the coupling of quiescence in metazoans to growth and to the structuring of tissues during development. I emphasize studies in the organism rather than in tissue culture, because these expose the diversity of regulation. While quiescence is likely to be a primitive biological process, it appears that in adapting quiescence to its many distinct biological settings, evolution has diversified it. Consideration of quiescence in different models gives us an overview of this diversity.

Coordinated regulation of niche and stem cell precursors by hormonal signaling

In the developing Drosophila ovary, the ecdysone signaling pathway controls the differentiation of both niche and germ line stem cell precursors.

Stem cells and their niches constitute units that act cooperatively to achieve adult body homeostasis. How such units form and whether stem cell and niche precursors might be coordinated already during organogenesis are unknown. In fruit flies, primordial germ cells (PGCs), the precursors of germ line stem cells (GSCs), and somatic niche precursors develop within the larval ovary. Together they form the 16–20 GSC units of the adult ovary. We show that ecdysone receptors are required to coordinate the development of niche and GSC precursors. At early third instar, ecdysone receptors repress precocious differentiation of both niches and PGCs. Early repression is required for correct morphogenesis of the ovary and for protecting future GSCs from differentiation. At mid-third instar, ecdysone signaling is required for niche formation. Finally, and concurrent with the initiation of wandering behavior, ecdysone signaling initiates PGC differentiation by allowing the expression of the differentiation gene bag of marbles in PGCs that are not protected by the newly formed niches. All the ovarian functions of ecdysone receptors are mediated through early repression, and late activation, of the ecdysone target gene broad. These results show that, similar to mammals, a brain-gland-gonad axis controls the initiation of oogenesis in insects. They further exemplify how a physiological cue coordinates the formation of a stem cell unit within an organ: it is required for niche establishment and to ensure that precursor cells to adult stem cells remain undifferentiated until the niches can accommodate them. Similar principles might govern the formation of additional stem cell units during organogenesis.

Tissue-specific stem cells reside in specialized microenvironments (niches). How the generation of niche cells and resident stem cells is coordinated, and how their correct numerical ratios are regulated, remains poorly understood. Here, we examine the potential mechanisms of this process by analyzing the formation of the fly ovary. Specifically, we uncover a role for ecdysone, which is a steroid hormone renowned for its role in insect molting but that also controls many aspects of larval development in flies and mammals. We find that ecdysone signaling in fly larvae coordinates the development of niche cells relative to their resident germ line stem cells (GSCs). Ecdysone receptors initially serve as repressors of differentiation, allowing precursor cells of both niches and GSCs time to proliferate and attain correct cell numbers. Later, ecdysone receptors allow differentiation of niches while simultaneously maintaining GSC precursors in an undifferentiated state, until the newly formed niches can accommodate them. Finally, ecdysone induces the differentiation of GSC precursors that are not incorporated in niches. Our work exemplifies one possible solution to coordinating stem cell and niche development: using a common signal to affect both cell types. A further understanding of these and other mechanisms will offer novel insights into regeneration and could help guide cell-based therapies for various diseases.

RNA polymerase II kinetics in polo polyadenylation signal selection

Regulated alternative polyadenylation is an important feature of gene expression, but how gene transcription rate affects this process remains to be investigated. polo is a cell-cycle gene that uses two poly(A) signals in the 3' untranslated region (UTR) to produce alternative messenger RNAs that differ in their 3'UTR length. Using a mutant Drosophila strain that has a lower transcriptional elongation rate, we show that transcription kinetics can determine alternative poly(A) site selection. The physiological consequences of incorrect polo poly(A) site choice are of vital importance; transgenic flies lacking the distal poly(A) signal cannot produce the longer transcript and die at the pupa stage due to a failure in the proliferation of the precursor cells of the abdomen, the histoblasts. This is due to the low translation efficiency of the shorter transcript produced by proximal poly(A) site usage. Our results show that correct polo poly(A) site selection functions to provide the correct levels of protein expression necessary for histoblast proliferation, and that the kinetics of RNA polymerase II have an important role in the mechanism of alternative polyadenylation.

Nonautonomous apoptosis is triggered by local cell cycle progression during epithelial replacement in Drosophila

Tissue remodeling involves collective cell movement, and cell proliferation and apoptosis are observed in both development and disease. Apoptosis and proliferation are considered to be closely correlated, but little is known about their coordinated regulation in physiological tissue remodeling in vivo. The replacement of larval abdominal epidermis with adult epithelium in Drosophila pupae is a simple model of tissue remodeling. During this process, larval epidermal cells (LECs) undergo apoptosis and are replaced by histoblasts, which are adult precursor cells. By analyzing caspase activation at the single-cell level in living pupae, we found that caspase activation in LECs is induced at the LEC/histoblast boundary, which expands as the LECs die. Manipulating histoblast proliferation at the LEC/histoblast boundary, either genetically or by UV illumination, indicated that local interactions with proliferating histoblasts triggered caspase activation in the boundary LECs. Finally, by monitoring the spatiotemporal dynamics of the S/G₂/M phase in histoblasts in vivo, we found that the transition from S/G₂ phases is necessary to induce nonautonomous LEC apoptosis at the LEC/histoblast boundary. The replacement boundary, formed as caspase activation is regulated locally by cell-cell communication, may drive the dynamic orchestration of cell replacement during tissue remodeling.

The steroid hormone ecdysone functions with intrinsic chromatin remodeling factors to control female germline stem cells in Drosophila

Steroid hormones are known systemic regulators of multiple normal and cancerous tissues; however, whether or how they impact the fate and function of adult stem cells is unclear. In the Drosophila ovary, insulin signals modulate the proliferation and self-renewal of germline stem cells (GSCs), yet despite evidence that additional systemic factors control GSC activity, these have remained largely unknown. Here, we report that ecdysone, a steroid hormone structurally related to mammalian sex steroids, directly regulates adult GSC proliferation and self-renewal independently of insulin signaling. Ecdysone controls GSCs through a functional interaction with the chromatin remodeling factors ISWI, an intrinsic epigenetic factor required for GSC fate and activity, and Nurf301, the largest subunit of the ISWI-containing NURF chromatin remodeling complex. Our findings support a link between systemic steroid hormones and the intrinsic chromatin remodeling machinery as a potential mechanism to promote broad transcriptional programs required for adult stem cell self-renewal.

Drosophila EGFR pathway coordinates stem cell proliferation and gut remodeling following infection

Background

Gut homeostasis is central to whole organism health, and its disruption is associated with a broad range of pathologies. Following damage, complex physiological events are required in the gut to maintain proper homeostasis. Previously, we demonstrated that ingestion of a nonlethal pathogen, Erwinia carotovora carotovora 15, induces a massive increase in stem cell proliferation in the gut of Drosophila. However, the precise cellular events that occur following infection have not been quantitatively described, nor do we understand the interaction between multiple pathways that have been implicated in epithelium renewal.

Results

To understand the process of infection and epithelium renewal in more detail, we performed a quantitative analysis of several cellular and morphological characteristics of the gut. We observed that the gut of adult Drosophila undergoes a dynamic remodeling in response to bacterial infection. This remodeling coordinates the synthesis of new enterocytes, their proper morphogenesis and the elimination of damaged cells through delamination and anoikis. We demonstrate that one signaling pathway, the epidermal growth factor receptor (EGFR) pathway, is key to controlling each of these steps through distinct functions in intestinal stem cells and enterocytes. The EGFR pathway is activated by the EGF ligands, Spitz, Keren and Vein, the latter being induced in the surrounding visceral muscles in part under the control of the Janus kinase/signal transducer and activator of transcription (JAK/STAT) pathway. Additionally, the EGFR pathway synergizes with the JAK/STAT pathway in stem cells to promote their proliferation. Finally, we show that the EGFR pathway contributes to gut morphogenesis through its activity in enterocytes and is required to properly coordinate the delamination and anoikis of damaged cells. This function of the EGFR pathway in enterocytes is key to maintaining homeostasis, as flies lacking EGFR are highly susceptible to infection.

Conclusions

This study demonstrates that restoration of normal gut morphology following bacterial infection is a more complex phenomenon than previously described. Maintenance of gut homeostasis requires the coordination of stem cell proliferation and differentiation, with the incorporation and morphogenesis of new cells and the expulsion of damaged enterocytes. We show that one signaling pathway, the EGFR pathway, is central to all these stages, and its activation at multiple steps could synchronize the complex cellular events leading to gut repair and homeostasis.

Dpp signaling directs cell motility and invasiveness during epithelial morphogenesis

Tissue remodeling in development and disease involves the coordinated invasion of neighboring territories and/or the replacement of entire cell populations. Cell guidance, cell matching, transitions from passive to migratory epithelia, cell growth and death, and extracellular matrix remodeling all impinge on epithelial spreading. Significantly, the extracellular signals that direct these activities and the specific cellular elements and mechanisms regulated by these signals remain in most cases to be identified. To address these issues, we performed an analysis of histoblasts (Drosophila abdominal epithelial founder cells) on their transition from a dormant state to active migration replacing obsolete larval epidermal cells (LECs). We found that during expansion, Decapentaplegic (Dpp) secreted from surrounding LECs leads to graded pathway activation in cells at the periphery of histoblast nests. Across nests, Dpp activity confers differential cellular behavior and motility by modulating cell-cell contacts, the organization and activity of the cytoskeleton, and histoblast attachment to the substrate. Furthermore, Dpp also prevents the premature death of LECs, allowing the coordination of histoblast expansion to LEC delamination. Dpp signaling activity directing histoblast spreading and invasiveness mimics transforming growth factor-beta and bone morphogenetic proteins' role in enhancing the motility and invasiveness of cancer cells, resulting in the promotion of metastasis.

A Drosophila insulin-like peptide promotes growth during nonfeeding states

In metazoans, tissue growth relies on the availability of nutrients--stored internally or obtained from the environment--and the resulting activation of insulin/IGF signaling (IIS). In Drosophila, growth is mediated by seven Drosophila insulin-like peptides (Dilps), acting through a canonical IIS pathway. During the larval period, animals feed and Dilps produced by the brain couple nutrient uptake with systemic growth. We show here that, during metamorphosis, when feeding stops, a specific DILP (Dilp6) is produced by the fat body and relays the growth signal. Expression of DILP6 during pupal development is controlled by the steroid hormone ecdysone. Remarkably, DILP6 expression is also induced upon starvation, and both its developmental and environmental expression require the Drosophila FoxO transcription factor. This study reveals a specific class of ILPs induced upon metabolic stress that promotes growth in conditions of nutritional deprivation or following developmentally induced cessation of feeding.

Interaction between Drosophila bZIP proteins Atf3 and Jun prevents replacement of epithelial cells during metamorphosis

Epithelial sheet spreading and fusion underlie important developmental processes. Well-characterized examples of such epithelial morphogenetic events have been provided by studies in Drosophila, and include embryonic dorsal closure, formation of the adult thorax and wound healing. All of these processes require the basic region-leucine zipper (bZIP) transcription factors Jun and Fos. Much less is known about morphogenesis of the fly abdomen, which involves replacement of larval epidermal cells (LECs) with adult histoblasts that divide, migrate and finally fuse to form the adult epidermis during metamorphosis. Here, we implicate Drosophila Activating transcription factor 3 (Atf3), the single ortholog of human ATF3 and JDP2 bZIP proteins, in abdominal morphogenesis. During the process of the epithelial cell replacement, transcription of the atf3 gene declines. When this downregulation is experimentally prevented, the affected LECs accumulate cell-adhesion proteins and their extrusion and replacement with histoblasts are blocked. The abnormally adhering LECs consequently obstruct the closure of the adult abdominal epithelium. This closure defect can be either mimicked and further enhanced by knockdown of the small GTPase Rho1 or, conversely, alleviated by stimulating ecdysone steroid hormone signaling. Both Rho and ecdysone pathways have been previously identified as effectors of the LEC replacement. To elicit the gain-of-function effect, Atf3 specifically requires its binding partner Jun. Our data thus identify Atf3 as a new functional partner of Drosophila Jun during development.